Five miles south of Tavistock, Buckland Abbey was a Cistercian monastery converted into a country house after the dissolution and most famously owned by Sir Francis Drake. The National Trust runs it, and the great barn, the Tudor remodelling of the church, and the small but rewarding collection of Drake memorabilia make it worth half a day.
The grounds are quietly beautiful: orchards, a herb garden, and woodland paths down to the Tavy. In recent years the property hit the news when a painting in storage was authenticated as a self-portrait by Rembrandt, now on permanent display. Combine a visit with a walk along the river back toward Tavistock if you have the time.